1. Field of the Invention
The invention relates to a semiconductor device and a fabrication method thereof, and more particularly to a thin film transistor and a fabrication method thereof.
2. Description of Related Art
In general, a liquid crystal display (LCD) mainly includes a thin film transistor (TFT) array substrate, a color filter substrate, and a liquid crystal layer sandwiched between the two substrates. An amorphous silicon TFT or a low temperature polysilicon TFT is usually adopted as a switch device of each sub-pixel in the TFT array substrate. In recent years, it is reported that, the oxide semiconductor TFT has relatively high carrier mobility in comparative with the amorphous silicon TFT. In addition, compared with the low temperature polysilicon TFT, the oxide semiconductor TFT has advantages of large area manufacturing and low manufacturing cost. As such, the oxide semiconductor TFT has high potential in development and may become the key device in the next generation of flat panel displays.
However, the stability of the oxide semiconductor TFT is likely to be influenced by external moisture and diffusion of hydrogen ions in the passivation layer. The existing passivation layer for mass production is usually formed by the plasma enhanced chemical vapor deposition process (PECVD), and the oxide semiconductor may be doped with hydrogen ions when the plasma is dissociated. As a result, the threshold voltage shift is increased. On the other hand, if a non-hydrogen film is formed by PVD, the yield is decreased because the sputtering rate of the film is low, and the capacitance coupling between the metal layers is generated due to high dielectric constant of the non-hydrogen film. Moreover, during the etching process, an etching undercut may occur in the passivation layer and the underlying gate insulating layer, causing the discontinuous formation of the pixel electrode on the passivation layer.